The first estrus cycle usually occurs by age 6-12 months; for some small breeds, as early as 5 months, and for some large and giant breeds, the first cycle may not occur until 14 months of age or older. On average, dogs have two cycles a year.
The estrus cycle lasts on average 12-21 days, but maybe be as short as a few days to four weeks. The estrus period length varies widely between breeds and individual dogs.
The length of a cycle varies widely, even for dogs of the same breed. If in doubt, assume the longer end of the range for the cycle length.
Bleeding occurs prior to a female being receptive to a male (allowing mounting by the male), but male dogs will be very attracted to the female in the proestrus stage.
Dogs can get pregnant during their first heat cycle, but this is not advisable as a 6-month old dog is not yet fully grown/mature, and complications for the mother and the puppies are more likely.
The first sign of your dog starting her heat cycle is a swollen vulva and bloody discharge from the vulva.
Typically, when your dog has her first heat, it will be a "split heat" in which she will develop a swollen vulva and have bloody discharge but she won't actually ovulate (release eggs) or let male dogs mount her. But about 2 to 6 weeks later you'll probably notice that she's in heat again; this is your dog's first real full heat cycle.
